In an imaging system utilizing the sublimation of dyes under the influence of heat from laser beams, beams are focused on the donor/receiver wrapped around a drum. Due to the high energy requirement of the sublimation process of typical dyes, it is advantageous to write with multiple laser beams to attain acceptable printing times.
As shown in U.S. Pat. No. 5,109,460 (S. Baek et al.), issued Apr. 28, 1992, a laser diode printing system has been built using a multiple line printhead with simple spot forming optics that is scanned across the image. The system is composed of a drum, a lead screw and a printhead. The optical system of the printhead is composed of a silicon V-grooved block for aligning a fiber array, an imaging lens and laser diodes. Multiple laser beams from one end of the fibers of the printhead are focused on a dye donor layer by a relaying lens. The printhead module is carried parallel to the drum axis in synchronization with the drum rotation. Multiple lines are written either helically or in a stepping-and-stare fashion to cover the whole imaging area. Image data is fed in proper synchronization with the drum rotation as well as the printhead module movement.
One alternative to the fiber array printhead is a laser diode array which can be individually addressed and modulated. See U.S. Pat. No. 4,804,975, issued to K. Yip, Kodak, entitled "Thermal Dye Transfer Apparatus Using Semiconductor Diode Laser Array". The advantage of using the laser diode array as the light source is that it can deliver more laser power to the donor at potentially a fraction of the cost of the fiber array printhead. When a laser is coupled to a fiber, about 50% of laser power is lost due to the mismatch of Numerical Aperture (N.A.) of the Emitting beam and the N. A. of the fiber (or acceptance angle of the fiber). Without the fiber coupling loss, the optical system efficiency of the laser diode array printhead can be more than 80%. The cost of the laser diode array is more expensive than a single element laser diode, but it is a lot less expensive than the cost of the equal number of single element laser diodes.
Disadvantages of the laser diode array are that the system may have a more complicated optical system and laser calibration problems due to a thermal cross-talk. Generally laser diodes emit an elliptical beam with a large aspect ratio. This makes the optical system more complicated than in the case of a fiber optic printhead system. In addition, a large field of view is required in the relay optical system to cover more than one element if a laser diode array is used in the system. Due to the difficulties mentioned above, the laser diode array has not been used as a light source for a printing system.
The reliability of a system using many active components depends upon the failure rates of each active component cascaded. Since a laser diode array has multiple elements on the same substrate, the reliability of the array is reduced substantially. This low reliability can be a serious problem to a printing system using the array. There are several ways to improve the reliability of the laser diode array system including employing a longer burn-in time for the lasers prior to installation in the equipment, and/or derating the operating power to the array.
Another potential difficulty in using the laser diode array in a laser thermal printer is the thermal cross-talk between adjacent elements in the array. Thermal cross-talk is a very difficult problem especially for a high quality image printing system.
It is well known that the output of a laser diode is very sensitive to the operating temperature of the active region. There are two ways to overcome this difficulty; temperature control and current control. The temperature control method controls the operating temperature of the active region to maintain it at as constant a temperature as possible by actively heating or cooling the laser diode. A thermoelectric temperature controller is often used for this purpose. The current control method controls the output power of the laser diode to maintain the output of the diode constant by adjusting the driving current by a close-loop feedback current driver. This method is more complicated and expensive. In some applications, the combination of two methods is used for better control of the output power of a laser diode.
According to another approach described in U.S. Pat. No. 5,151,915 issued Sep. 29, 1992 to Paoli, to eliminate the thermal variations that inherently accompany modulation of a laser's output by varying its driving current, the laser output is modulated by varying the voltage applied to a modulator region to operate the lasers at a somewhat elevated but constant temperature.
In using a laser diode array, it is not easy to apply the two thermal compensating methods described above. The spacing between the elements is so close that the operating condition of adjacent elements can influence the thermal equilibrium conditions of the neighboring elements. The laser diode array emits several closely spaced beams making the monitoring of optical power for each element extremely difficult. There is therefore, a need for an improved method to compensate the thermal cross-talk in a laser diode array which accounts for all possible operating conditions of the neighboring elements.